Stresses to plants may be caused by both biotic and abiotic agents. For example, biotic causes of stress include infection with pathogen, insect feeding, and parasitism by another plant such as mistletoe. Abiotic stresses include, for example, excessive or insufficient available water, nitrogen, temperature extremes, and synthetic chemicals such as herbicides.
Abiotic stresses such as drought, high salinity and deficiency of nutrient elements adversely affect the growth and productivity of plants including crops, which significantly limit crop production worldwide. Cumulatively, these factors are estimated to be responsible for an average 70% reduction in agricultural production. Plants are sessile and have to adjust to the prevailing environmental conditions of their surroundings. This has led to their development of a great plasticity in gene regulation, morphogenesis, and metabolism. Adaptation and defense strategies involve the activation of genes encoding proteins important in the acclimation or defense towards the different stressors.
The absorption of nitrogen by plants plays an important role in their growth (Gallais et al., J. Exp. Bot. 55(396):295-306 (2004)). Plants synthesize amino acids from inorganic nitrogen in the environment. Consequently, nitrogen fertilization has been a powerful tool for increasing the yield of cultivated plants, such as rice, maize and soybean. Lack of sufficient plant-available nitrogen for optimum growth and development may be considered as an abiotic stress. In order to avoid pollution by nitrates and to maintain a sufficient profit margin, today farmers desire to reduce the use of nitrogen fertilizer. If a plant variety has increased nitrogen assimilation capacity, it would also be expected to have increased growth and yield. In summary, plant varieties that have better nitrogen use efficiency (NUE) are desirable.
Activation tagging can be utilized to identify genes with the ability to affect a trait. This approach has been used in the model plant species Arabidopsis thaliana (Weigel et al., Plant Physiol. 122:1003-1013 (2000)). Insertions of transcriptional enhancer elements can dominantly activate and/or elevate the expression of nearby endogenous genes. This method can be used to identify genes of interest for a particular trait (e.g. nitrogen use efficiency in a plant), that when placed in an organism as a transgene, can alter that trait.